1. Field of the Invention
The invention relates to an electronic circuit for bidirectional conversion of a high input voltage to a direct-current output voltage with indirect coupling, more specifically, an electronic circuit for use in a power supply system for rail vehicles.
2. Related Art
Nowadays, 16.7 Hz or 50 Hz transformers are utilized in rail vehicles to convert the high voltage of 15 kV or of 25 kV respectively in the overhead lines of a railroad system to the voltage of nearly 1000 V, whereby the value does not exceed the rating of silicon power components which are able to drive the electric motors. These transformers are disadvantageous because of their overall weight. This weight occasions loss of energy, which is undesirable since energy costs have to be saved to an ever increasing extent in rail vehicles as well. Their size, furthermore, considerably reduces the useful space available as complementary components may also be required to cope with other direct current voltage power supplies (1.5 kV DC, 3 kV DC).
The published application DE 198 17 752 A1 describes a feeding circuit for feeding an electric driver unit that supplies a plurality of drive groups including at least one driving motor and one input converter. Each input converter (thus fed by the equivalent ratio of the input voltage) is provided with a transformer to operate the driving motors of the separate secondary direct-current voltage circuits with different loads. To achieve that the load is uniformly distributed to the input converters of the individual drive groups, a compensating circuit which electrically links the driving converters together is provided. The choice of the described low fundamental frequencies of the transformer suggests that the transformer converters are operated in a non-resonant mode.
The circuit disadvantageously has an additional compensating circuit which is necessary because the converter sections are submitted to different loads. This makes the system even more complicated which increases the failure probability. Due to the hard switch operation of the transformer converters, the efficiency of the system is comparatively low. Taking the low frequency into account, the totality of the individual transformers is heavy in comparison to a comparable single transformer. It is also required to use as many motors as converters in order to be able to cope with the high voltage. This may lead to a significantly higher weight than conventional traction drivers (especially with 25 kV).
At least one of published German application 198 27 872 and European Patent No. 820 893 A2 describes a driving circuit for a rail vehicle consisting of mains series connection and of secondary parallel connection of converter sections, each converter section being provided with at least one transformer. Several secondary converter sections are furthermore utilized.
Taking into account the growth law for transformers, and due to the plurality of individual transformers, the circuit has the disadvantage that it has a high overall weight and that it involves accordingly higher costs compared to a system with but one transformer. The several secondary converter sections furthermore require considerable expenditure.
The publication Carpita M. et al, “Preliminary Design of a 18 kV locomotive”, 19 Sep. 1995, EPE'95, 6th European Conference on Power Electronics and Applications, Sevilla, Sep. 19–21, 1995, Vol. 2, Page(s) 2.153–2.158, European Power Electronics and Drive's Association XP000537737 describes a feeder system for operating rail vehicles with 18 kV direct-current voltage systems. This system comprises a transformer with several associated converter sections. The transformers are allocated one or several secondary rectifiers. The operation of this circuit is unidirectional, i.e., the energy only flows from the overhead line to the vehicle drive so it is not possible to recover the breaking energy. Another drawback is that the input voltage of the primary converters of the transformer is directly determined by the overhead line. As a result thereof, operation in the resonant mode cannot be realized if the direct-current output voltage is to remain stable. Therefore, operation is hard switching with high losses and accordingly low efficiency. Furthermore, a change in the configuration topology for operation with an alternating voltage or with lower dc voltage is not possible.